The present invention generally relates to hydraulic systems of the types used in machinery, including but not limited to machines having multiple functions performed by one or more hydraulic circuits. More particularly, this invention relates to hydraulic systems that contain one or more positive displacement units capable of operating as pumps and motors, and distributed valve systems that, in combination with the positive displacement unit(s), can be used to control multiple actuators of multi-function machinery.
Compact excavators, wheel loaders and skid-steer loaders are examples of multi-function machines whose operations involve controlling movements of various implements of the machines. FIG. 1 illustrates a compact excavator 100 as having a cab 101 mounted on top of an undercarriage 102 via a swing bearing (not shown) or other suitable device. The undercarriage 102 includes tracks 103 and associated drive components, such as drive sprockets, rollers, idlers, etc. The excavator 100 is further equipped with a blade 104 and an articulating mechanical arm 105 comprising a boom 106, a stick 107, and an attachment 108 represented as a bucket, though it should be understood that a variety of different attachments could be mounted to the arm 105. The functions of the excavator 100 include the motions of the boom 106, stick 107 and bucket 108, the offset of the arm 105 during excavation operations with the bucket 108, the motion of the blade 104 during grading operations, the swing motion for rotating the cab 101, and the left and right travel motions of the tracks 103 during movement of the excavator 100. In the case of a compact excavator 100 of the type represented in FIG. 1, the blade 104, boom 106, stick 107, bucket 108 and offset functions are typically powered with linear actuators, represented as hydraulic cylinders 109 through 114 in FIG. 1, while the travel and swing functions are typically powered with rotary hydraulic motors (not shown in FIG. 1).
On conventional excavators, the control of these functions is accomplished by means of directional control valves. However, throttling flow through control valves is known to waste energy. In some current machines, the rotary functions (rotary hydraulic drive motors for the tracks 103 and rotary hydraulic swing motor for the cabin 101) are realized using displacement control (DC) systems, which notably exhibit lower power losses and allow energy recovery. In contrast, the position and velocity of the linear actuators 109-114 for the blade 104, boom 106, stick 107, bucket 108, and offset functions typically remain controlled with directional control valves. It is also possible to control linear hydraulic actuators directly with hydraulic pumps. Several pump-controlled configurations are known, using both constant and variable displacement pumps. Displacement control of linear actuators with single rod cylinders has been described in U.S. Pat. No. 5,329,767, DE000010303360A1, EP000001588057A1 and WO 2004/067969, and offers the possibility of large reductions in energy requirements for hydraulic actuation systems. Other aspects of using displacement control systems can be better appreciated from further reference to Zimmerman et al., “The Effect of System Pressure Level on the Energy Consumption of Displacement Controlled Actuator Systems,” Proc. of the 5th FPNI PhD Symposium, Cracow, Poland, 77-92 (2008), and Williamson et al., “Efficiency Study of an Excavator Hydraulic System Based on Displacement-Controlled Actuators,” Bath ASME Symposium on Fluid Power and Motion Control (FPMC2008), 291-307 (2008).
Various efforts have examined the use of integrated valve systems to improve the performance of hydraulic systems, including hydraulic systems of types that can be adapted for use in the excavator 100 of FIG. 1. For example, J. Andruch and J. Lumkes, “A Hydraulic System Topography with Integrated Energy Recover and Reconfigurable Flow Paths Using High Speed Valves,” Proceedings of the 51st National Conference on Fluid Power (NCFP), NCFP 108-24.1, 649-657 (March 2008), reports research that was conducted to explore how digital valves can be used to recapture energy when connected as a network of valves and actuators with a single pump. This system, designated a “topography with integrated energy recovery,” or TIER™ system, is schematically represented with reference numeral 10 in FIG. 2. The system 10 is represented as comprising a pair of hydraulic actuators 12 and 14, a prime mover (motor) 16, a fixed displacement pump 18 (operating only in a pumping mode), and four sets 20, 22, 24 and 26 of electronically-operated on/off valves. Each valve set 20, 22, 24 and 26 is connected to one of the ports of the actuators 12 and 14. A high pressure conduit system 28 fluidically connects the pump 18 to a first valve of each set 20, 22, 24 and 26. Furthermore, a low pressure conduit system 30 fluidically connects a reservoir 38 to a second valve of each set 20, 22, 24 and 26. A particular aspect of the system 10 is the inclusion of a third conduit system 32, referred to as a secondary pressure rail (SPR), that enables the ports of each actuator 12 and 14 to be fluidically connected to either the high or low pressure conduit system 28 or 30. These connections to the third conduit system 32 are controlled through the operation of a pair of valves 34 and 36.
The TIER™ system 10 represented in FIG. 2 can be operated in a manner similar to an electrical system neural network that has been adapted to hydraulic systems. As known in the art, neural network controllers are able to adapt and learn, or be trained, during operation. In combination with an appropriate electronic control system, the TIER™ system is capable of similar behavior by enabling two or more hydraulic actuators (such as the actuators 12 and 14) to reconfigure themselves, in other words, to sum flows from multiple changing sources, isolate faulty fluid conduit sections, and adaptively change operating modes (load sensing, IMV, displacement control, and modes unique to the TIER™ system). The system 10 can operate similarly to an independent metering valve (IMV), while also offering the ability to perform flow regeneration on linear actuators with different piston areas. As known in the art, flow regeneration refers to the situation in which both sides of a piston within a hydraulic cylinder are exposed to the same pressure, such that the effective area of the cylinder is the cross-sectional area of the piston rod. Flow regeneration enables increased actuating speeds because the flow required to extend the cylinder is only the change in volume of the piston rod within the cylinder.
In addition to traditional flow regeneration, the TIER™ system 10 is able to provide flow regeneration between two or more actuators within the system 10 through the use of the third conduit system (SPR) 32, which enables the system 10 to transfer flow from an assistive load to a resistive load. As used herein, the term “assistive load” refers to operating conditions in which the desired movement of a hydraulic actuator and the load applied to the actuator are in the same direction, for example, when a hydraulic actuator (cylinder) is lowering a large mass. In contrast, “resistive load” is used herein to denote conditions in which the external load applied to an actuator opposes the desired motion of the actuator, for example, when a hydraulic actuator is raising a load. In a conventional hydraulic system, as the articulating arm 105 is lowered the pressure within the side of the cylinder 109 opposite the piston rod would simply be throttled through a valve before being returned to the reservoir 38, leading to energy loss. In contrast, the TIER™ system 10 enables this high pressure fluid to flow to another actuator, for example, one of the other actuators 110-114 in which the high pressure flow from the cylinder 109 can be used to assist the operation of the other cylinder 110-114. When pressure/flow relationships of two or more actuators allow for regeneration, the SPR 32 of FIG. 2 can be used to recover energy and improve the cycle efficiency, for example, by about 33% compared to using industry standard spool valves in pressure-compensated load sensing systems.
Other configurations of hydraulic systems have been proposed to provide similar capabilities, for example, in WO 2008/009950, which discloses a digital pump/motor unit capable of both pumping and motoring with a system of digital valves.
Notwithstanding the above advancements, further improvements in hydraulic systems are desired, particularly for the purpose of realizing high performance energy-efficient hydraulic systems.